1. Field of the Invention
This invention relates in general to subsea oil and gas production, and in particular to equipment and methods for protection a subsea well assembly from the corrosive effects of salt water using electrical charge.
2. Prior Art
Components of a subsea well production system, including the associated production wellhead, tree, and manifold, are generally built from steel, requiring protection to prevent the corrosion in seawater. Sacrificial cathodic protection is often used to protect the steel components. To perform cathodic protection, either aluminum or zinc sacrificial anodes are attached to well components, and the anodes corrode to produce an electrical current that protects the steel from corrosion.
Corrosion in seawater is an electrochemical process. During the chemical reaction of metals with the environment to form corrosion products (such as rust on steel), metallic atoms give up one or more electrons to become positively charged ions, and oxygen and water combine to form negatively charged ions. The reactions occur at rates, which result in no charge buildup. All the electrons given up by the metal atoms are consumed by the other reaction. Cathodic protection is a process that prevents the corrosion reaction by creating an electric field so that current flows into the metal. This prevents the formation of metal ions by setting up a potential gradient at the surface, which opposes the electric current produced by flow of electrically charged ions away from the metal surface as the product of corrosion. The electric field must be of a strength to counter the field produced by the corrosion reaction to ensure that metal ions are not formed. A source of the electric field that opposes the corrosion reaction may be a current supplied from the preferential corrosion of a metal anode with different electrochemical properties in the environment, and which has a stronger anodic reaction with the environment than does the offshore structure. Thus, current flows to the structure from the anode, which itself progressively corrodes in preference to the structure. This technique is known as sacrificial anode cathodic protection.
Though sacrificial anode cathodic protection are well functioning for the prevention of corrosion of the well production system, there are some problems with the passive system. The anodes used in the system must be appropriately placed and distributed through-out the well production system, i.e., on various components of the Christmas tree, to ensure an appropriate electrical field is induced by the electro-chemical reaction. The addition of these anodes greatly adds to the weight of the Christmas tree structure. Also, the anodes are generally not operable over the life of the well, which may be in production for 50 or more years. Finally, currents can affect the efficacy of the sacrificial system. Accordingly, the condition of the anodes must be monitored and failing anodes must be periodically replaced, which can be difficult depending upon the location of the anodes.